Users Guide

Autonomous system areas
OSPF operates in a hierarchy. The largest entity within the hierarchy is the autonomous system (AS). The AS is a collection of
networks under a common administration that share a common routing strategy. OSPF is an intra-AS, Interior Gateway Routing
Protocol (IGRP) that receives routes from and sends routes to other AS.
You can divide an AS into several areas, which are groups of contiguous networks and attached hosts administratively grouped.
Routers with multiple interfaces can participate in multiple areas. These routers, called area border routers (ABRs), maintain
separate databases for each area. Areas are a logical grouping of OSPF routers that an integer or dotted-decimal number
identifies.
Areas allow you to further organize routers within the AS with one or more areas within the AS. Areas allow subnetworks to hide
within the AS—minimizing the size of the routing tables on all routers. An area within the AS may not see the details of another
area’s topology. An area number or the router’s IP address identifies AS areas.
Areas, networks, and neighbors
The backbone of the network is Area 0, also called Area 0.0.0.0, the core of any AS. All other areas must connect to Area 0.
An OSPF backbone distributes routing information between areas. It consists of all area border routers and networks not wholly
contained in any area and their attached routers.
The backbone is the only area with a default area number. You configure all other areas Area ID. If you configure two
nonbackbone areas, you must enable the B bit in OSPF. Routers, A, B, C, G, H, and I are the backbone, see Autonomous system
areas.
● A stub area (SA) does not receive external route information, except for the default route. These areas do receive
information from interarea (IA) routes.
● A not-so-stubby area (NSSA) can import AS external route information and send it to the backbone as type-7 LSA.
● Totally stubby areas are also known as no summary areas.
Configure all routers within an assigned stub area as stubby and do not generate LSAs that do not apply. For example, a Type
5 LSA is intended for external areas and the stubby area routers may not generate external LSAs. A virtual link cannot traverse
stubby areas.
Networks and neighbors
As a link-state protocol, OSPF sends routing information to other OSPF routers concerning the state of the links between them.
The Up or Down state of those links is important. Routers that share a link become neighbors on that segment. OSPF uses the
hello protocol as a neighbor discovery and keepalive mechanism. After two routers are neighbors, they may proceed to
exchange and synchronize their databases, which creates an adjacency.
Layer 3
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